Add fine-grained logging for long running transformers.
Every 10 seconds, log a message detailing the duration of the
transformer and any secondary assets it is waiting for.
These messages are made visible by the `--verbose` or `--verbosity=all`
flags.
BUG=
R=nweiz@google.com
Review URL: https://codereview.chromium.org//1353013003 .

Barback is an asset build system. It is the library underlying pub's asset transformers in pub build and pub serve.

Given a set of input files and a set of transformations (think compilers, preprocessors and the like), it will automatically apply the appropriate transforms and generate output files. When inputs are modified, it automatically runs the transforms that are affected.